Caption: Scanning electron micrographs (SEMs) of various kinds of marine diatoms: (top row, left to right): Actinoptychus, Biddulphia; (bottom row, left to right): Biddulphia, Aulacodiscus. The diatoms are a group of photosynthetic, single-celled algae containing about 100,000 species. They form an important part of the plankton at the base of the marine and freshwater food chains. The characteristic feature of diatoms is their intricately patterned, glass- like cell wall, or frustule. The frustule often has rows of tiny holes, known as striae. Magnification ranging from x1600 to x2200.